The complete structures of the surface antigens from 5 of the 31 serovars in the Mycobacterium avium-M. intracellulare-M. scrofulaceum (MAIS) complex have been established. That from serovar 25 has the structure: Fatty Acyl-Phe-aThr-Ala-Alaninol- (3,4-di-O-Me-Rha) 6-dTal-alpha(2 yields 1) -Rha alpha(3 yields 1)-2-Me-Fucp alpha(4 yields 1)-2-Me-Fuc. In other serovars the Fatty Acyl-Peptidyl-3,4-di-O-Me-Rha is retained whereas the oligosaccharide is peculiar to each serovar. The oligosaccharides from serovar 8 and 9 have the respective structures: 4,6-Carboxethylidene-3-Me-Glcp -beta(1 yields 3)-Rhap- alpha(1 yields 2)-6-Deoxytal-2,3-Me2 -Fucp- alpha(1 yields 4) -2,3-Me2-Fucp- alpha(1 yields 3) -Rhap- alpha(1 yields 2)-6-Deoxytal-. These type of structures are not present in Mycobacterium kansasii and several other mycobacteria. Instead the surface antigen from M. kansasii consists of an acylated glycopeptide, containing glucose and xylose. Electron microscopy of smooth colony mycobacteria showed a superficial layer of filaments, composed in the majority of the peptidoglycolipid antigens. This filamentous capsule may be an important factor in the pathogenicity of atypical mycobacteria. Proposed research will center on the biosynthesis and further structural probing of these antigens. The immunogenicity of these will be examined in the hope of preparing monospecific antisera. The possibility that the PGL antigens of MAIS serovars are the products of lysogenic conversion will also be explored.